1. Field of the Invention
This invention relates to a sintered ceramic material based on silicon nitride (Si.sub.3 N.sub.4) which has superior cutting effects for use in machining and a process for producing the material.
2. Prior Art
Ceramics based on silicon nitride have excellent properties such as high strength and hardness at high temperatures and, therefore, they have been used in high temperature structural components.
Recently, some proposals have been made for applying such ceramics to cutting tools. Most of the proposals are concerned with additives to improve their sintering characteristics and strength. Such additives include the following materials: aluminium oxide (Al.sub.2 O.sub.3), magnesium oxide (MgO), yttrium oxide (Y.sub.2 O.sub.3) and the like. These additives were used as sintering aids to improve the sintering characteristics. Titanium carbide (TiC), titanium nitride (TiN), titanium carbide-titanium nitride solid solution (TiC,N), tungsten carbide (WC), tantalium carbide (TaC), and tantalium nitride (TaN) were used as additives to enhance the refractoriness abrasion resistance and toughness of sintered silicon nitride.
Ceramic cutting tools sintered with such additives, especially, titanium carbide and/or titanium nitride systems, are known to have excellent characteristics in the aspects of refractoriness, abrasion resistance and toughness. Many proposals have hitherto been made concerning such materials.
For instance, Japanese Laid-Open Patent Publication No. (Sho) 56-32,377 employs aluminium nitride (AlN), alumina (Al.sub.2 O.sub.3), magnesia (MgO), and silica (SiO.sub.2) as additives for silicon nitride material for cutting tools. About 5 to 40 percent by weight of one or two or more of titanium carbide, titanium nitride, and titanium carbide-titanium nitride solid solution are also added to improve its toughness.
Japanese Laid-Open Patent Publication No. (Sho) 58-73,670 (U.S. Pat. No. 4,280,973) discloses a process for preparing a high performance cutting tool using compositions based on silicon nitride with additives such as yttrium oxide (Y.sub.2 O.sub.3), magnesia (MgO), cerium oxide (CeO.sub.2) and zirconium oxide (ZrO.sub.2), tungsten (W) and titanium carbide. This patent publication is associated with the following other Japanese Laid-Open Publications: Nos. (Sho) 57-205,374, 57-205,376, 58-95,662, 58-199,782, and 58-213,679 (U.S. Pat. No. 4,425,141).
As disclosed in the publications aforementioned, the cutting tools based on silicon nitride containing titanium carbide are known to have excellent cutting properties as compared with those containing titanium nitride or other additives. However, the silicon nitride tools dispersed with titanium carbide have several disadvantages in that their sinterability is relatively poor due to the formation of by-products during the sintering of the materials which produces less-densified tools.
Therefore, in the field of the relevant industries, the cutting tools have long been desired in which the prior art problems have been effectively eliminated. The applicants have now discovered after intensive research that it is possible to obtain a cutting tool having excellent performance by treating the titanium carbide additive in a special manner which eliminates such serious problems during the sintering. The present invention therefore is based on this discovery.